This section provides background information related to the present disclosure which is not necessarily prior art.
According to a United Nations (UN) report, with current energy usage (largely based on fossil fuels), the Earth's temperature will rise between 1.5 to 4° C. over the next 50 years due to greenhouse gases. With this rise in temperature, Earth faces a future of extreme weather, rising sea levels and melting polar ice from soaring levels of carbon dioxide, methane and other greenhouse gases. A global effort in green manufacturing and use of renewable energy resources is necessary to reduce emission to prevent temperatures from rising; preventing catastrophic and dangerous disruptions worldwide. In parallel to this challenge, a quarter of the world's population (roughly 1.6 billion) has no electricity. Lack of availability of electricity has a direct correlation to poverty and poor health. The current approach of transporting energy to remote locations is difficult and expensive. Thus, production of energy from local resources is necessary. Use of photovoltaics (solar cells) and their manufacturing play an important role in the improvement of environment and society.
Photovoltaic (PV) devices directly convert sunlight into electricity. On average, the sun illuminates the Earth with more than 10,000 times the light energy humans currently consume, PVs have the potential to be a large and environmental friendly energy source. The production cost of solar electricity has reduced over the last decade and can now compete with grid electricity. This increase is driven by advances in PV technology, large scale manufacturing, and also because the relative cost of oil and coal has increased over the last decade. Crystalline silicon (c-Si) based PVs currently dominate the market due to their large efficiency and low cost of production. This domination is also largely because Si is available in abundance, it is non-toxic and it is stable under harsh environment conditions. Further advances in this technology are needed to make it compatible with green manufacturing, achieve higher efficiency and to reduce overall cost.
One of the biggest obstacles to harvesting renewable energy sources is cost. Many approaches to harvesting solar energy are being explored by the scientific community (e.g., solar cells, thermal concentrators, etc.). Among the many viable approaches, solar cells are attractive as they provide direct conversion from light to electric energy. Both organic and inorganic materials are being explored for the design of high efficiency low-cost solar cells. Organic solar cells, although having the potential to reduce cost, still suffer from long term stability. Inorganic materials for solar cell production, the most commonly used being Gallium Arsenide (GaAs), silicon (Si), Indium Phosphide (InP), and cadmium telluride, have shown to have long term stability while still maintaining the potential to reduce cost. Reducing the cost of producing solar electricity plays an important role in spreading its use as a clean and renewable energy source. The cost to manufacture solar cells can be reduced in two ways, by increasing their efficiency and by developing low-cost production technologies. Among these device technologies, silicon (Si) devices are attractive as the base material. Silicon is available in abundance in nature and thin-films of Si can be deposited using a variety of techniques, such as e-beam, plasma enhanced chemical vapor deposition, and pulsed laser deposition. Silicon based technologies provide an optimum balance between material cost, efficiency and product life time.
The reason Si solar cells have not become popular in daily life is because existing solar cell processes are very elaborate and require expensive fabrication steps. The number of mask steps required to fabricate solar cells can be large and minimizing these steps is necessary to reduce cost. Furthermore, for high efficiency solar cell technologies, the cost associated with the manufacture of Si wafers is high due to required chemical and mechanical polishing. Among the many types of solar cell device technologies, silicon heterojunction (SHJ) solar cells have become popular due to their large efficiencies and ability to process at low temperatures at reduced cost. FIG. 1 shows a cross sectional diagram of a SHJ solar cell. The SHJ consists of thin hydrogenated amorphous silicon (a-Si:H) wide bandgap buffer layers deposited on crystalline silicone (c-Si) wafers. These hydrogenated buffer layers (a-Si:H) help improve the quality of the material and provide a higher bandgap layer. A transparent conductive oxide (TCO) layer is deposited on the highly doped a-Si:H layers. Top and bottom electrode layers are screen printed. This design enables energy conversion efficiency above 20% at the production level. The key feature of this technology is that the metal contacts, which have a highly recombination active interface, are separated from the absorption region by utilizing wide bandgap a-Si:H layers. This enables high open-circuit voltages typically associated with heterojunction devices. Pyramid structures (5-10 μm) are included to enhance capture of incoming light (anti-reflection, AR, layer). The heterojunction solar cells are also utilized to enhance the absorption of a wider optical spectrum. The efficiency of Si solar cells increases by growing a top layer of a semiconductor (a-Si:H) with an energy bandgap larger than the c-Si layer. Incident photons with energy greater than the bandgap energy of a-Si:H will be absorbed in this top layer. Photons of lower energy will be absorbed in the c-Si layer to form carrier pairs. In this way, more of the energy of the solar spectrum is used to generate electrical power.
From a processing point of view, the key advantages of SHJ technology is that thin Si wafers can be used and an overall fewer number of fabrication steps are required compared to other Si based devices. The pyramidal texture is formed using a mixture of potassium hydroxide (KOH) and isopropyl alcohol (IPA) solutions at an elevated temperature. Texturing helps by lowering external optical reflection, and in parallel, improves internal reflection, which improves light trapping. Such texturing can be formed on polished single-crystal silicon, though in recent years methods of forming them on multi-crystalline silicon have been developed. The a-Si:H layers are deposited in vacuum using chemical vapor deposition using silane gas. Prior to the deposition of a-Si:H layers, the c-Si is cleaned to minimize interfacial defects. Oxide and nitride layers are deposited to passivate the surface as a passivation layer. This passivation layer is patterned prior to screen printing conductive electrodes on both sides. Although the above solar cell design minimizes many process steps compared to conventional silicon solar cells while achieving high efficiency, the expensive steps remaining are the substrate material (c-Si), vacuum deposition of a-Si:H layers, and wet etching of textured surface. Thus, there is a need to develop a manufacturing process that: i) avoids the use of vacuum deposition, ii) does not require polished Si substrates, iii) avoids the use of corrosive chemicals to achieve antireflection (AR) texturing, and iv) further enhances conversion efficiency. Here, a silicon carbide (SiC) based heterojunction device processed using a laser as an approach to meet the above challenges is provided.